In forthcoming evolutions of cellular radio communication system standards, such as Long Term Evolution (LTE) and High-Speed Packet Access (HSPA), the maximum data rate will surely be higher than in previous systems. Higher data rates typically require larger system channel bandwidths. For an IMT advanced system (i.e., a “fourth generation” (4G) mobile communication system), bandwidths of 100 megahertz (MHz) and larger are being considered.
LTE and HSPA are sometimes called “third generation” communication systems and are currently being standardized by the Third Generation Partnership Project (3GPP). The LTE specifications can be seen as an evolution of the current wideband code division multiple access (WCDMA) specifications. An IMT advanced communication system uses an internet protocol (IP) multimedia subsystem (IMS) of an LTE, HSPA, or other communication system for IMS multimedia telephony (IMT). The 3GPP promulgates the LTE, HSPA, WCDMA, and IMT specifications, and specifications that standardize other kinds of cellular wireless communication systems.
An LTE system uses orthogonal frequency division multiplex (OFDM) as a multiple access technique (called OFDMA) in the downlink (DL) from system nodes to user equipments (UEs). An LTE system has channel bandwidths ranging from about 1 MHz to 20 MHz, and supports data rates up to 100 megabits per second (Mb/s) on the largest-bandwidth channels. One type of physical channel defined for the LTE downlink is the physical downlink shared channel (PDSCH), which conveys information from higher layers in the LTE protocol stack and is mapped to one or more specific transport channels. The PDSCH and other LTE channels are described in 3GPP Technical Specification (TS) 36.211 V8.4.0, Physical Channels and Modulation (Release 8) (September 2008), among other specifications.
In an OFDMA communication system like LTE, the data stream to be transmitted is portioned among a number of narrowband subcarriers that are transmitted in parallel.
In general, a resource block devoted to a particular UE is a particular number of particular subcarriers used for a particular period of time. A resource block is made up of resource elements (REs), each of which is a particular subcarrier used for a smaller period of time. Different groups of subcarriers can be used at different times for different users. Because each subcarrier is narrowband, each subcarrier experiences mainly flat fading, which makes it easier for a UE to demodulate each subcarrier. Like many modern communication systems, DL transmissions in an LTE system are organized into frames of 10 milliseconds (ms) duration, and each frame typically includes twenty successive time slots. OFDMA communication systems are described in the literature, for example, U.S. Patent Application Publication No. US 2008/0031368 A1 by B. Lindoff et al.
FIG. 1 depicts a typical cellular communication system 10. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. In general, each RNC directs calls to and from a UE, such as a mobile station (MS), mobile phone, or other remote terminal, via appropriate base station(s) (BSs), which communicate with each other through DL (or forward) and uplink (UL, or reverse) channels. In FIG. 1, RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26.
Each BS, or enodeB in LTE vocabulary, serves a geographical area that is divided into one or more cell(s). In FIG. 1, BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26, although a sector or other area served by signals from a BS can also be called a cell. In addition, a BS may use more than one antenna to transmit signals to a UE. The BSs are typically coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. The RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
It should be understood that the arrangement of functionalities depicted in FIG. 1 can be modified in LTE and other communication systems. For example, the functionality of the RNCs 12, 14 can be moved to the enodeBs 22, 24, 26, and other functionalities can be moved to other nodes in the network. It will also be understood that a base station can use multiple transmit antennas to transmit information into a cell/sector/area, and those different transmit antennas can send respective, different pilot signals.
Mobility is an important functionality in cellular communication systems like an LTE system. Fast and efficient cell searches and received signal measurements are important for a UE to get and stay connected to a suitable cell, which can be called a “serving cell”, and to be handed over from one serving cell to another. On a regular basis, a UE measures its received signal strength and signal quality of each detected cell, including the serving cell, to determine whether a handover to a new cell is needed or not. The new cell can be on the same frequency as the serving cell or on a different frequency.
In an LTE system, handover decisions are based on measurements of reference signal received power (RSRP), which can be defined as the average UE-received signal power of reference symbols (RS) transmitted by an enodeB. A UE measures RSRP on its serving cell as well as on neighboring cells that the UE has detected as a result of a cell search procedure, as specified for example in Section 5.2 of 3GPP TS 36.304 V8.4.0, User Equipment (UE) Procedures in Idle Mode (Release 8) (December 2008).
The RS, or pilots, are transmitted from each Node B at known frequencies and time instants, and are used by UEs for synchronization and other purposes besides handover. Such reference signals and symbols are described for example in Sections 6.10 and 6.11 of 3GPP TS 36.211 cited above. RS are transmitted from each of possibly 1, 2, or 4 transmit antennas of an enodeB on particular REs that can be conveniently represented on a frequency-vs.-time plane as depicted in FIG. 2. It will be understood that the arrangement of FIG. 2 is just an example and that other arrangements can be used.
FIG. 2 shows an arrangement of subcarriers in resource blocks in two successive time slots, which can be called a sub-frame, in an LTE system. The frequency range depicted in FIG. 2 includes twenty-seven subcarriers, only nine of which are explicitly indicated. In FIG. 2, the resource blocks, which are indicated by dashed lines, each include twelve subcarriers spaced apart by fifteen kilohertz (kHz), which together occupy 180 kHz in frequency and 0.5 ms in time, or one time slot. FIG. 2 shows each time slot including seven OFDM symbols, or REs, each of which has a short (normal) cyclic prefix, although six OFDM symbols having long (extended) cyclic prefixes can be used instead in a time slot. It will be understood that resource blocks can include various numbers of subcarriers for various periods of time.
RS transmitted by a first transmit (TX) antenna of a Node B are denoted R and by a possible second TX antenna in the node are denoted by S. In FIG. 2, RS are depicted as transmitted on every sixth subcarrier in OFDM symbol 0 and OFDM symbol 4 (because the symbols have short cyclic prefixes) in every slot. Also in FIG. 2, the RSs in symbols 4 are offset by three subcarriers relative to the RS in OFDM symbol 0, the first OFDM symbol in a slot.
Besides reference signals, predetermined synchronization signals are needed during cell search. LTE uses a hierarchical cell search scheme similar to WCDMA, in which synchronization acquisition and cell group identifier are obtained from different synchronization channel (SCH) signals. Thus, a primary synchronization channel (P-SCH) signal and a secondary synchronization channel (S-SCH) signal are defined with a pre-defined structure in Section 6.11 of 3GPP TS 36.211. For example, P-SCH and S-SCH signals can be transmitted on particular subcarriers in particular time slots. In an LTE system, the enodeBs transmit two different synchronization signals: a primary synchronization signal (PSS) and a secondary synchronization signal (SSS). Primary and secondary synchronization signals are described in U.S. Patent Application Publication No. US 2008/0267303 A1 by R. Baldemair et al.
In an LTE system, RSRP is estimated with OFDM symbols that include RSs, and a received signal strength indicator (RSSI) should also be measured with the OFDM symbols that are used for the RSRP measurement. FIG. 2 shows the SSS and PSS as OFDM symbols 5, 6 (assuming operation with the short cyclic prefix and frequency-division duplex (FDD). Current LTE systems have the PSS and SSS symbols transmitted in the middle six resource blocks in sub-frames 0 and 5.
FIG. 2 also indicates by the four vertical arrows on the time axis the OFDM symbols that are used for RSRP and RSSI measurements.
While RSRP indicates received signal strength, reference signal received quality (RSRQ) is an implicit measure of the load on the cell, as seen by the UE, and so RSRQ can be an important measure for the network to use in making good handover decisions. RSRQ can be defined as the ratio of the measured RSRP to the measured RSSI. In general, RSSI is the total received signal power over a predetermined number of resource blocks used for signal quality measurements.
Improving energy efficiency in the base station (network) has recently received attention. To reduce cost for a network operator, it is useful to reduce the power consumption of base stations, especially in low-load conditions. One way to do that is to use discontinuous transmission (DTX) in the enodeBs, which is to say that when a cell has no load or a low load, the enodeB spends some of its time in a low-power “sleep” mode with a certain duty cycle.
Nevertheless, an enodeB cannot “sleep” all of the time because it needs to transmit signals that enable UEs to find it and synchronize themselves to it, as well as signals used for handover measurement purposes. One way to increase the DTX possibilities and at the same time provide good handover performance is to use the synchronization signals also for handover measurements, as described in, for example, U.S. Patent Application Publication No. US 2007/0297324 A1 by B. Lindoff et al. In an LTE system, the reference signals, which are transmitted in at least four OFDM symbols in every resource block as depicted in FIG. 2, are used for handover measurements based on RSRP.
Therefore, there is a need for improved methods and apparatus that use synchronization signals in carrying out received-signal measurements for handover and other purposes.